1. Field of the Invention
This invention relates to solution mining of subterranean formations of water soluble salt deposits utilizing a solvent at high temperatures thereby heating the formation and recapturing heat given up thereto. It more particularly relates to solution mining of subterranean formations of soluble salt deposits which have increasing solubilities and increasing rates of dissolution with increasing temperatures, e.g., potassium chloride.
2. Description of the Prior Art
Subterranean deposits of water soluble salts have been solution mined by introducing an aqueous solvent through one or more boreholes communicating with the deposit and withdrawing a solution enriched in the dissolved salt. A cavity is created thereby and begins to grow as larger surface areas of the deposit are exposed to the solvent which in turn dissolves more soluble salt until the cavity becomes so large that the soluble salt can no longer be extracted at commercially attractive rates or surface subsidence becomes a risk. Salts such as sylvinite, trona, and halite have been extracted from subterranean deposits employing this technique.
It is often advantageous to extract these soluble salts using aqueous solvents hotter than the deposit to effect relatively rapid extraction rates as well as to provide a solvent having a greater capacity than a cooler solvent. For example, U.S. Pat. No. 2,161,800 to Cross teaches solution mining of potassium compounds by circulating through underground potash beds super heated water or brine unsaturated with respect to the potassium compounds at temperatures of about 200.degree. C. or above. While this method uses high temperature solvents which have the benefit of causing the potassium compounds to be extracted relatively rapidly, there is no provision in the method to recover the heat transferred to the underground potash beds. Thus, since potash beds are relatively highly heat conducting, a sizable expense is incurred in heat loss thereto.
U.S. Pat. No. 3,050,290 to Caldwell teaches a method of circulating a solvent through an underground trona formation which is thereby heated to between 50.degree. C. and about 200.degree. C. by heating the solvent before each cycle. A portion of the circulating solvent is bled from the system for purposes of extracting sodium values therefrom. This method minimizes the amount of heat necessarily supplied to heat up the solution in the passage or cavity and to heat the surrounding trona formation, but again, no provision is made to recover heat loss to the formation by transient conduction. Therefore, the cost for the substantial and rapid heat input required to bring the solution and surrounding cavity to the desired dissolving temperature is never recaptured.
U.S. Pat. No. 3,278,234 to Helvenston teaches a method of solution mining potassium chloride by feeding solvent between 50.degree. C. and 100.degree. C. into a subterranean potassium chloride and sodium chloride bearing deposit without losing heat by maintaining withdrawn enriched solution within 15.degree. C. of the matural deposit temperature. However, this method does not have the advantage of increased potassium chloride dissolution at temperatures substantially higher than the natural formation temperature. Consequently, there is a compromise in the amount of potassium chloride mined per unit volume of solvent used.
U.S. Pat. No. 3,348,883 to Jacoby teaches the concomitant solution mining and refining of soluble minerals, such as sodium chloride ore containing sulfates as an impurity, by utilizing a high temperature geological environment to satisfy the heat source requirement to selectively exclude the sulfates during mining. This method does not involve introducing heat into the soluble mineral deposits beneath the earth's surface, hence any inefficiency in heat utilization does not result in a sizable loss in energy costs. However, the geological environment of most subterranean soluble salt deposits is not conducive to this method. Thus, the application of this method is limited.
U.S. Pat. No. 4,074,754 to Christian teaches a method of producing geothermal energy and/or minerals from subterranean reservoirs at about 600.degree. F. and containing up to 250,000 ppm salt content by injecting into the reservoir a low salinity water at ambient surface temperature. The injected water is allowed to become heated after which water is withdrawn from the reservoir containing heat energy and minerals. Here again, the application of this method is limited as is the Jacoby method described above.
It is therefore a desideratum that subterranean soluble salt deposits be recovered using a high temperature solvent whereby heat lost to the subterranean deposit can be somehow recaptured.